A model for degradation of electrochemical devices based on linear non-equilibrium thermodynamics and its application to lithium ion batteries

Transport through ionic conducting membranes is examined. An equation describing the chemical potential, μs, of electrically neutral species, s, in the membrane is derived in terms of ionic and electronic currents, and ionic and electronic transport resistances. It is shown that the μs in the membrane need not be mathematically bounded by the values at the two electrodes (reservoirs) if the ionic and the electronic currents through the membrane are in the same direction. Conditions could develop under which the μs in the membrane may exceed the thermodynamic stability of the membrane even when exposed to stable conditions at the two electrodes. It is shown that during charging, chemical potential of lithium, μLi, in the electrolyte of a lithium-ion battery may exceed that corresponding to pure lithium thus causing lithium precipitation and/or reaction with the electrolyte. It is also shown that in a lithium ion battery pack containing several cells, degradation may occur during discharge due to cell imbalance. In unbalanced cells, the SEI layer may form at both the anode/electrolyte and the cathode/electrolyte interfaces. A bi-layer separator comprising an electronic conductor and an electronic insulator is proposed for improved stability of lithium batteries.

► Analyze multi-species transport in electrochemical systems.
► Demonstrate that in membranes, electronic conductivity cannot be set to zero to ensure local equilibrium.
► Demonstrate chemical potentials of electrically neutral species in the membrane need not be limited by values at electrodes if ionic and electronic currents are in the same direction.
► Show chemical potential of lithium in electrolyte of a lithium ion battery can exceed values at anode causing lithium precipitation. This causes degradation and capacity loss of lithium ion batteries.